I'm born and living in Berlin. I spent part of my youth in Hanover. The dialect of German is basically the most normal German langue, Hanover has no dialect by definition.
It never even crossed my mind that it could be in any way negative to answer somebody in English; Maybe I'm biased, but I fundamentally believe that it would be if very high positive impact if humans would speak a common language. That would certainly not be German.
I understand that there are people who like to learn German. I am very aware that it is a good thing to learn multiple languages. Of course, if you want to live in German speaking regions, it makes sense to learn it - but I think many people here are happy that they have an opportunity to practice English. It is in no way negative if somebody answers in English. Auf die Idee komme ich gar nicht erst! And if you let the person know that you like to practice German, anybody would happily do that - if he know it. We think speaking English is a good, valuable thing.
(To Germans reading this: let me know if you object, I know I'm somewhat biased, I'm willing to correct this answer.)
Again, I was surprised that it is even possible that answering in English could be received in a negative way. That's certainly not intended. I would feel answering in English is being polite. Let the person know that you like answers in German, I don't have a proposal how to do that best, but use any way whatsoever!